Functional decreases in hydraulic and mechanical properties of field-grown transgenic poplar trees caused by modification of the lignin synthesis pathway through downregulation of the 4-coumarate:coenzyme A ligase gene

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Functional decreases in hydraulic and mechanical properties of field-grown transgenic poplar trees caused by modification of the lignin synthesis pathway through downregulation of the 4-coumarate:coenzyme A ligase gene

Abstract:

Society uses massive quantities of wood fiber in production of paper, and demand for fiber is projected to increase further as production of biofuels from fermentation of plant cellulosic materials increases. Because these end uses generally require the costly step of removing of lignin, wood with reduced or more easily extracted lignin has long been a goal of plant biotechnologists. However, there is little information on how reduction of lignin affects wood properties, tree development, and survival, especially in field environments. We studied a gene that had been previously reported to substantially reduce lignin content and improve biomass production in poplar. An antisense, xylem expressed version of the Pt4CL1 gene that encodes 4-coumarate:coenzyme A ligase (4CL) was inserted into hybrid poplar (Populus tremula x alba, INRA 717-1B4) and the growth and physiology of 14 transgenic lines (i.e., independent gene insertion events) was assessed over two growing seasons. Transgenic lines had 30-70% reductions in 4CL RNA expression in young shoots. This corresponded to 5-45% reduction in lignin as indicated by total monomer release through thioacidolysis and/or nitrobenzene oxidation. Only three transgenic lines with modest (>10%) reductions in lignin content sustained adequate growth and had normal tree form. Trees from five lines with severely reduced lignin formed up 24-60% of the stem cross-sectional area in brown colored wood that was essentially non-conductive to water, presumably due to the ectopic deposition of non-lignin phenolics and associated tyloses that occluded vessels. Across all genotypes, the transgenic lines had up to a three-fold increase in tensionwood, 40% lower modulus of elasticity, 25% lower modulus of rupture, 45% reduced resistance to xylem embolism (P₅₀), and a 60% increase in stem taper. Comparable patterns in wood density in lines that were lower in lignin by up to 9% by mass was compensated for by a 3% increase in polysaccharide content associated with tensionwood and a 6% increase in the deposition of extractives. Taken together, these data suggest that extensive field testing, ecophysiology, and wood quality evaluations are critical components of research and development on lignin-modified tree crops.